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          Codeforces Round #407 (Div. 2)
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        <p>感觉这场题目出的很不错 <span id="more"></span></p>
<p><a target="_blank" rel="noopener" href="http://codeforces.com/contest/789/problem/A">A. Anastasia and pebbles</a> 边界情况没考虑好，WA了一次，感觉算是"比较难"的一个A题</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;bits/stdc++.h&gt;</span></span></span><br><span class="line"><span class="keyword">using</span> <span class="keyword">namespace</span> std;</span><br><span class="line"><span class="keyword">int</span> n,k;</span><br><span class="line"><span class="keyword">int</span> w[<span class="number">100010</span>];</span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">	<span class="built_in">scanf</span>(<span class="string">&quot;%d%d&quot;</span>,&amp;n,&amp;k);</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=n;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="built_in">scanf</span>(<span class="string">&quot;%d&quot;</span>,&amp;w[i]);</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">int</span> ans=<span class="number">0</span>;</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=n;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		w[i]=<span class="built_in">max</span>(w[i],<span class="number">0</span>);</span><br><span class="line">		<span class="keyword">if</span>(w[i]&gt;=<span class="number">2</span>*k)</span><br><span class="line">		&#123;</span><br><span class="line">			ans+=w[i]/(<span class="number">2</span>*k);</span><br><span class="line">			w[i]%=<span class="number">2</span>*k;</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">if</span>(w[i]&gt;k)</span><br><span class="line">		&#123;</span><br><span class="line">			ans++;</span><br><span class="line">			w[i]=<span class="number">0</span>;</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">else</span> <span class="keyword">if</span>(w[i]!=<span class="number">0</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			ans++;</span><br><span class="line">			w[i+<span class="number">1</span>]-=k;</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	cout&lt;&lt;ans&lt;&lt;endl;</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p><a target="_blank" rel="noopener" href="http://codeforces.com/contest/789/problem/B">B. Masha and geometric depression</a> 这个B真的恶心...WA了大概四五次才过...下次应该把这种边界条件都列在纸上，防止写一写把之前想到的忘了</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;bits/stdc++.h&gt;</span></span></span><br><span class="line"><span class="keyword">using</span> <span class="keyword">namespace</span> std;</span><br><span class="line"><span class="keyword">long</span> <span class="keyword">long</span> b1,q,l,m;</span><br><span class="line"><span class="keyword">long</span> <span class="keyword">long</span> a[<span class="number">100010</span>];</span><br><span class="line">map&lt;<span class="keyword">long</span> <span class="keyword">long</span>,<span class="keyword">int</span>&gt; ha;</span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">	<span class="built_in">scanf</span>(<span class="string">&quot;%lld%lld%lld%lld&quot;</span>,&amp;b1,&amp;q,&amp;l,&amp;m);</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=m;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="built_in">scanf</span>(<span class="string">&quot;%lld&quot;</span>,&amp;a[i]);</span><br><span class="line">		ha[a[i]]=<span class="number">1</span>;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">if</span>(<span class="built_in">abs</span>(b1)&gt;l)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="built_in">printf</span>(<span class="string">&quot;0\n&quot;</span>);</span><br><span class="line">		<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">if</span>(q==<span class="number">0</span>)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">int</span> ans=<span class="number">0</span>;</span><br><span class="line">		<span class="keyword">if</span>(ha.<span class="built_in">find</span>(<span class="number">0</span>)==ha.<span class="built_in">end</span>())</span><br><span class="line">		&#123;</span><br><span class="line">			<span class="built_in">printf</span>(<span class="string">&quot;inf\n&quot;</span>);</span><br><span class="line">			<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">else</span></span><br><span class="line">		&#123;</span><br><span class="line">			<span class="keyword">if</span>(ha.<span class="built_in">find</span>(b1)==ha.<span class="built_in">end</span>()&amp;&amp;<span class="built_in">abs</span>(b1)&lt;=l) ans++;</span><br><span class="line">			<span class="built_in">printf</span>(<span class="string">&quot;%d\n&quot;</span>,ans);</span><br><span class="line">			<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">if</span>(b1==<span class="number">0</span>)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">if</span>(ha.<span class="built_in">find</span>(<span class="number">0</span>)==ha.<span class="built_in">end</span>())</span><br><span class="line">		&#123;</span><br><span class="line">			<span class="built_in">printf</span>(<span class="string">&quot;inf\n&quot;</span>);</span><br><span class="line">			<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">else</span></span><br><span class="line">		&#123;</span><br><span class="line">			<span class="built_in">printf</span>(<span class="string">&quot;0\n&quot;</span>);</span><br><span class="line">			<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">if</span>(<span class="built_in">abs</span>(q)==<span class="number">1</span>)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">int</span> ans=<span class="number">0</span>;</span><br><span class="line">		<span class="keyword">if</span>(ha.<span class="built_in">find</span>(b1)==ha.<span class="built_in">end</span>()&amp;&amp;q==<span class="number">1</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			<span class="built_in">printf</span>(<span class="string">&quot;inf\n&quot;</span>);</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">else</span> <span class="keyword">if</span>(ha.<span class="built_in">find</span>(-b1)==ha.<span class="built_in">end</span>()&amp;&amp;q==<span class="number">-1</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			<span class="built_in">printf</span>(<span class="string">&quot;inf\n&quot;</span>);</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">else</span> <span class="keyword">if</span>(ha.<span class="built_in">find</span>(b1)==ha.<span class="built_in">end</span>()&amp;&amp;q==<span class="number">-1</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			<span class="built_in">printf</span>(<span class="string">&quot;inf\n&quot;</span>);</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">else</span> <span class="built_in">printf</span>(<span class="string">&quot;%d\n&quot;</span>,ans);</span><br><span class="line">		<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">int</span> ans=<span class="number">0</span>;</span><br><span class="line">	<span class="keyword">while</span>(<span class="number">1</span>)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">if</span>(<span class="built_in">abs</span>(b1)&gt;l)</span><br><span class="line">		&#123;</span><br><span class="line">			<span class="built_in">printf</span>(<span class="string">&quot;%d\n&quot;</span>,ans);</span><br><span class="line">			<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">if</span>(ha.<span class="built_in">find</span>(b1)==ha.<span class="built_in">end</span>())</span><br><span class="line">		&#123;</span><br><span class="line">			ans++;</span><br><span class="line">		&#125;</span><br><span class="line">		b1*=q;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p><a target="_blank" rel="noopener" href="http://codeforces.com/contest/789/problem/C">C. Functions again</a> 稍微处理一下，就是一个最大子段和的问题，基础dp，但是这个问题是子段和问题的变种，需要处理一下开端点的奇偶性。</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;bits/stdc++.h&gt;</span></span></span><br><span class="line"><span class="keyword">using</span> <span class="keyword">namespace</span> std;</span><br><span class="line"><span class="keyword">long</span> <span class="keyword">long</span> a[<span class="number">110000</span>];</span><br><span class="line"><span class="keyword">long</span> <span class="keyword">long</span> b[<span class="number">110000</span>];</span><br><span class="line"><span class="keyword">long</span> <span class="keyword">long</span> c[<span class="number">110000</span>];</span><br><span class="line"><span class="keyword">long</span> <span class="keyword">long</span> ans1[<span class="number">110000</span>];</span><br><span class="line"><span class="keyword">long</span> <span class="keyword">long</span> ans2[<span class="number">110000</span>];</span><br><span class="line"><span class="keyword">long</span> <span class="keyword">long</span> ans;</span><br><span class="line"><span class="keyword">int</span> n;</span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">	<span class="built_in">scanf</span>(<span class="string">&quot;%d&quot;</span>,&amp;n);</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=n;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="built_in">scanf</span>(<span class="string">&quot;%lld&quot;</span>,&amp;a[i]);</span><br><span class="line">	&#125;</span><br><span class="line">	ans=<span class="number">-1e18</span>;</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;n;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		ans=<span class="built_in">max</span>(<span class="built_in">abs</span>(a[i]-a[i+<span class="number">1</span>]),ans);</span><br><span class="line">		<span class="keyword">if</span>(i%<span class="number">2</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			b[i]=<span class="built_in">abs</span>(a[i]-a[i+<span class="number">1</span>]);</span><br><span class="line">			c[i]=-b[i];</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">else</span></span><br><span class="line">		&#123;</span><br><span class="line">			c[i]=<span class="built_in">abs</span>(a[i]-a[i+<span class="number">1</span>]);</span><br><span class="line">			b[i]=-c[i];</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">int</span> a1=<span class="number">0</span>;</span><br><span class="line">	<span class="keyword">int</span> a2=<span class="number">0</span>;</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;n;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">if</span>(i%<span class="number">2</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			ans1[i]=<span class="built_in">max</span>(ans1[i<span class="number">-1</span>]+b[i],b[i]);</span><br><span class="line">			<span class="keyword">if</span>(i!=<span class="number">1</span>) ans2[i]=ans2[i<span class="number">-1</span>]+c[i];</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">else</span></span><br><span class="line">		&#123;</span><br><span class="line">			ans2[i]=<span class="built_in">max</span>(ans2[i<span class="number">-1</span>]+c[i],c[i]);</span><br><span class="line">			ans1[i]=ans1[i<span class="number">-1</span>]+b[i];</span><br><span class="line">		&#125;</span><br><span class="line">		ans=<span class="built_in">max</span>(ans,<span class="built_in">max</span>(ans1[i],ans2[i]));</span><br><span class="line">	&#125;</span><br><span class="line">	cout&lt;&lt;ans&lt;&lt;endl;</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p><a target="_blank" rel="noopener" href="http://codeforces.com/contest/789/problem/D">D. Weird journey</a> 出的很好的一道题。</p>
<blockquote>
<p>给定一个无向图，问有多少种走法，使得所有边都经过，且只有两条边经过一次，其余每条边都经过两次。两种走法不同当且仅当经过一次的两条边的边集不同，无重边，有自环</p>
</blockquote>
<p>因为是无向图，我们访问任意一个子图，使得这个子图的每条边都被访问两次，那这个访问的起点和终点一定重合（可以把无向边拆成两条有向边，然后就是个欧拉回路）。知道了这个之后，如果这个图没有自环的话，那么这两个只访问一次的边，一定相邻。 然后自环其实跟在这个点停留是一样的，那么自环可以和任意一条边（包括自环）进行组合就好了。 这个题的坑点是可以有散点存在，只要边联通就好了，比赛的时候就是因为这个，一直WA18</p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;bits/stdc++.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">define</span> N 1000101  </span></span><br><span class="line"><span class="keyword">using</span> <span class="keyword">namespace</span> std;</span><br><span class="line"><span class="keyword">long</span> <span class="keyword">long</span> du[N];</span><br><span class="line"><span class="keyword">int</span> fa[N];</span><br><span class="line"><span class="keyword">int</span> h[N];</span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">ff</span><span class="params">(<span class="keyword">int</span> x)</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">	<span class="keyword">if</span>(fa[x]==x) <span class="keyword">return</span> x;</span><br><span class="line">	fa[x]=<span class="built_in">ff</span>(fa[x]);</span><br><span class="line">	<span class="keyword">return</span> fa[x];</span><br><span class="line">&#125;</span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">	<span class="keyword">int</span> n,m;</span><br><span class="line">	<span class="built_in">scanf</span>(<span class="string">&quot;%d%d&quot;</span>,&amp;n,&amp;m);</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=n;i++) fa[i]=i;</span><br><span class="line">	<span class="keyword">long</span> <span class="keyword">long</span> t=<span class="number">0</span>;</span><br><span class="line">	<span class="keyword">long</span> <span class="keyword">long</span> o=<span class="number">0</span>;</span><br><span class="line">	<span class="keyword">int</span> e;</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=m;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">int</span> x,y;</span><br><span class="line">		<span class="built_in">scanf</span>(<span class="string">&quot;%d%d&quot;</span>,&amp;x,&amp;y);</span><br><span class="line">		<span class="keyword">if</span>(x==y) h[x]++,o++;</span><br><span class="line">		<span class="keyword">else</span> du[x]++,du[y]++,t++;</span><br><span class="line">		e=x;</span><br><span class="line">		<span class="keyword">if</span>(<span class="built_in">ff</span>(x)!=<span class="built_in">ff</span>(y))</span><br><span class="line">		&#123;</span><br><span class="line">			fa[<span class="built_in">ff</span>(x)]=<span class="built_in">ff</span>(y);</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=n;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">if</span>(<span class="built_in">ff</span>(i)!=<span class="built_in">ff</span>(e)&amp;&amp;(<span class="built_in">ff</span>(i)!=i||h[i]==<span class="number">1</span>))</span><br><span class="line">		&#123;</span><br><span class="line">			cout&lt;&lt;<span class="number">0</span>&lt;&lt;endl;</span><br><span class="line">			<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">long</span> <span class="keyword">long</span> ans=<span class="number">0</span>;</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=n;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		ans+=du[i]*(du[i]<span class="number">-1</span>)/<span class="number">2</span>;</span><br><span class="line">	&#125;</span><br><span class="line">	ans+=o*t;</span><br><span class="line">	ans+=o*(o<span class="number">-1</span>)/<span class="number">2</span>;</span><br><span class="line">	cout&lt;&lt;ans&lt;&lt;endl;</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p><a target="_blank" rel="noopener" href="http://codeforces.com/contest/789/problem/E">E. The Great Mixing</a> 本质上，就是取一些数字（取过的数字可以重复取），然后使他们的和等于<span class="math inline">\(0\)</span> 就是假设取了<span class="math inline">\(s_1,s_2,....,s_m\)</span>然后就是让<span class="math inline">\((s_1-n)+(s_2-n)+...+(s_m-n)=0\)</span> 这样的话，就有两种做法： 第一种是正常的dp,就是<span class="math inline">\(dp[i][j]\)</span>表示取<span class="math inline">\(i\)</span>个可乐，能不能凑出结果<span class="math inline">\(j\)</span>，可以用位运算进行优化，时间复杂度是<span class="math inline">\(O(N^3/32)\)</span></p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;bits/stdc++.h&gt;</span></span></span><br><span class="line"><span class="keyword">using</span> <span class="keyword">namespace</span> std;</span><br><span class="line">bitset&lt;2001&gt; dp[<span class="number">2</span>];</span><br><span class="line"><span class="keyword">bool</span> a[<span class="number">1100</span>]=&#123;&#125;;</span><br><span class="line"><span class="keyword">int</span> n,k;</span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">	<span class="built_in">scanf</span>(<span class="string">&quot;%d%d&quot;</span>,&amp;n,&amp;k);</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=k;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">int</span> t;</span><br><span class="line">		<span class="built_in">scanf</span>(<span class="string">&quot;%d&quot;</span>,&amp;t);</span><br><span class="line">		a[t]=<span class="number">1</span>;</span><br><span class="line">	&#125;</span><br><span class="line">	dp[<span class="number">0</span>][<span class="number">1000</span>]=<span class="number">1</span>;</span><br><span class="line">	<span class="keyword">int</span> no,la;</span><br><span class="line">	no=<span class="number">0</span>;</span><br><span class="line">	la=<span class="number">1</span>;</span><br><span class="line">	<span class="keyword">int</span> re;</span><br><span class="line">	<span class="keyword">for</span>(re=<span class="number">1</span>;re&lt;=<span class="number">1000</span>;re++)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="built_in">swap</span>(no,la);</span><br><span class="line">		dp[no].<span class="built_in">reset</span>();</span><br><span class="line">		<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">0</span>;i&lt;=<span class="number">1000</span>;i++)</span><br><span class="line">		&#123;</span><br><span class="line">			<span class="keyword">if</span>(a[i]) dp[no]|=(dp[la]&lt;&lt;i)&gt;&gt;n;</span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">if</span>(dp[no][<span class="number">1000</span>]==<span class="number">1</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			<span class="built_in">printf</span>(<span class="string">&quot;%d\n&quot;</span>,re);</span><br><span class="line">			<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="built_in">puts</span>(<span class="string">&quot;-1&quot;</span>);</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>第二种是每个可能的结果作为一个点，然后转移就是选择哪种可乐，在不同的点上转移，然后找到这个图上最小的环，用BFS就行。复杂度<span class="math inline">\(O(N^2)\)</span></p>
<figure class="highlight cpp"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">include</span><span class="meta-string">&lt;bits/stdc++.h&gt;</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">define</span> pb push_back</span></span><br><span class="line"><span class="keyword">using</span> <span class="keyword">namespace</span> std;</span><br><span class="line"><span class="keyword">bool</span> a[<span class="number">1100</span>];</span><br><span class="line"><span class="keyword">int</span> n,k;</span><br><span class="line"><span class="keyword">int</span> d[<span class="number">3100</span>];</span><br><span class="line"><span class="keyword">bool</span> v[<span class="number">3100</span>];</span><br><span class="line">vector&lt;<span class="keyword">int</span>&gt; u;</span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">main</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">	<span class="built_in">scanf</span>(<span class="string">&quot;%d%d&quot;</span>,&amp;n,&amp;k);</span><br><span class="line">	<span class="keyword">for</span>(<span class="keyword">int</span> i=<span class="number">1</span>;i&lt;=k;i++)</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">int</span> t;</span><br><span class="line">		<span class="built_in">scanf</span>(<span class="string">&quot;%d&quot;</span>,&amp;t);</span><br><span class="line">		<span class="keyword">if</span>(a[t]==<span class="number">0</span>)</span><br><span class="line">		&#123;</span><br><span class="line">			u.<span class="built_in">pb</span>(t-n);</span><br><span class="line">			a[t]=<span class="number">1</span>;</span><br><span class="line">		&#125;			</span><br><span class="line">	&#125;</span><br><span class="line">	queue&lt;<span class="keyword">int</span>&gt; q;</span><br><span class="line">	q.<span class="built_in">push</span>(<span class="number">1000</span>);</span><br><span class="line">	v[<span class="number">1000</span>]=<span class="number">1</span>;</span><br><span class="line">	<span class="keyword">while</span>(!q.<span class="built_in">empty</span>())</span><br><span class="line">	&#123;</span><br><span class="line">		<span class="keyword">int</span> t=q.<span class="built_in">front</span>();</span><br><span class="line">		q.<span class="built_in">pop</span>();</span><br><span class="line">		<span class="keyword">for</span>(<span class="keyword">auto</span> &amp;i:u)</span><br><span class="line">		&#123;	</span><br><span class="line">			<span class="keyword">if</span>(t+i&lt;<span class="number">0</span>||t+i&gt;<span class="number">2000</span>) <span class="keyword">continue</span>;</span><br><span class="line">			<span class="keyword">if</span>(t+i==<span class="number">1000</span>)</span><br><span class="line">			&#123;</span><br><span class="line">				<span class="built_in">printf</span>(<span class="string">&quot;%d\n&quot;</span>,d[t]+<span class="number">1</span>);</span><br><span class="line">				<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">			&#125;</span><br><span class="line">			<span class="keyword">if</span>(v[t+i]==<span class="number">0</span>)</span><br><span class="line">			&#123;</span><br><span class="line">				d[t+i]=d[t]+<span class="number">1</span>;</span><br><span class="line">				v[t+i]=<span class="number">1</span>;</span><br><span class="line">				q.<span class="built_in">push</span>(t+i);</span><br><span class="line">			&#125;</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="built_in">puts</span>(<span class="string">&quot;-1&quot;</span>);</span><br><span class="line">	<span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>实际写了一下，二者时间差距其实不大</p>

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